1. Field of the Invention
The present invention relates to a coated abrasive article and a process for producing the same.
2. Discussion of the Art
Coated abrasive articles (e.g., abrasive cloth and abrasive paper, etc.) are produced by applying a first adhesive on a backing (e.g., paper, cloth, etc.), applying abrasive grains to the first adhesive by electrostatic coating or gravity coating, curing the first adhesive with drying, applying a second adhesive over the first adhesive and abrasive grains, and then curing the second adhesive with drying.
In this case, as the second adhesive, for example, a synthetic resin (e.g., phenolic resin, epoxy resin, urea resin, etc.) or glue is used. Among the foregoing, a water soluble resol phenolic resin is widely used, because it is cheap and is superior in strength and heat resistance. However, the curing rate of the resol phenolic resin is low, and it requires long time for drying and curing. Further, when the resin is cured at a high temperature in order to accelerate drying and curing, boiling of water (as a solvent) as well as vapor of condensate formed on curing cause blisters (or little foams) in the cured resin layer, which results in a decrease of strength. As described above, it is difficult to reduce the drying time.
A relation between percentage of solids in and viscosity of typical resol phenolic resins commercially available in Japan for use as phenolic resins for abrasive cloth and paper is shown in FIG. 1. As is apparent from FIG. 1, resol phenolic resins containing about 65 to 82% solids and having a viscosity of about 700 to 10,000 cps are used for abrasive cloth and paper. Most popularly, water is used as a solvent. A resin in which alcoholic solvent (e.g., methanol, etc.) is partially or exclusively used is also commercially available.
After the second adhesive containing phenolic resin is applied, the second adhesive is cured by heating at a temperature of from about 50.degree. to 110.degree. C. for about 50 minutes to several hours in a festoon oven, which can contain hundreds or thousands meters of web. After the web is wound, the second adhesive is completely cured by heating to a temperature of about 60.degree. to 130.degree. C. for several hours to 20 hours.
On the other hand, in the case of using a short tunnel oven in place of the festoon oven, coating speed and drying speed must be remarkably decreased, and, therefore, productivity becomes low. Accordingly, it is difficult to reduce curing time with the current formulation and curing step.
To improve productivity, there is proposed a method for accelerating heating and curing by applying some modifications and additives to the resol phenolic resin. For example, there is a method comprising the steps of adding another resin to the phenolic resin to accelerate gelation, winding the coated abrasive product, and then curing the phenolic resin by after-cure (post-cure). In Japanese Patent Kokai No. 55-83573, there is proposed that a mixture of a water based resol and a condensate product which is made from a phenol component including phenol or polyfunctional phenol having high reactivity and formaldehyde be used as a binder. Further, a method for accelerating curing by acidifying the resol phenolic resin with an acid such as p-toluenesulfonic acid is also used.
However, the modification of resin or the acceleration of curing causes a decrease of crosslinking density, which results in deterioration of heat resistance. Further, hardness and strength are also decreased.
It would be desirable to shorten the curing time of the second adhesive and to remarkably improve productivity by increasing the molecular weight of a water soluble resol phenolic resin formulated in the second adhesive and selecting an organic solvent suitable for the resin.